The Greenbushes pegmatite deposit consists of a primary pegmatite intrusion with numerous smaller, generally linear pegmatite dikes and pods to the east. The primary intrusion and its subsidiary dikes and pods are concentrated within shear zones on the boundaries of granofels, ultramafic schists and amphibolites. The pegmatites are crosscut by ferrous-rich, mafic dolerite which is of paramount importance to the current mining methods. The pegmatite body is over 3 km long (north by northwest), up to 300 m wide (normal to dip), strikes north to north-west and dips moderately to steeply west to south-west. The syn-tectonic development of the pegmatite has given rise to mylonitic fabrics, particularly along host rock contacts.
The Greenbushes pegmatite is mineralogically segregated into five primary zones. Internally, the Greenbushes pegmatite consists of the Contact Zone, Potassium Feldspar (Potassium) Zone, Albite (Sodium) Zone, Mixed Zone and Spodumene (Lithium) Zone. The zones differ from many other rare-metal pegmatites in that they do not appear concentric, but are lenticular in nature, with inter-fingering along strike and down dip. They do not have a quartz core. The mine sequence was later subjected to the transgressive east-west dike and conformable sill dolerite intrusions.
The highest concentrations of primary ore minerals are found in specific mineralogical zones or assemblages within the pegmatite. The high-grade lodes within the main pegmatite body exhibit variable dips from 80 to 20° towards the west and south-west. Tantalum (tantalite) and tin (cassiterite) mineralization is concentrated in the Sodium Zone which is characterized by albite (Na-plagioclase), tourmaline and mica (muscovite). The Lithium Zone is enriched in the lithium bearing silicate spodumene. The mixed zone contains lower concentrations of tantalum and lithium. The final major zone is the potassium feldspar microcline which is not as economically important.
In general, the hanging wall to the pegmatite bodies is composed of amphibolite (meta-basalt and sub-volcanic intrusive bodies) whereas the footwall is granofels, dominantly of metasedimentary origin. The amphibolites and dolerites contain occasional stringers and pods of sulphides such as pyrite, pyrrhotite and chalcopyrite. Arsenopyrite and arsenolamprite (native arsenic) are noted in some areas, particularly within granofels and amphibolite inliers in the main pegmatite. Accessory minerals in the pegmatite are apatite and small amounts of beryl and garnet. Waste rocks are not as well-logged or understood at the same level of detail as the pegmatites in general.
Weathering and erosion of the pegmatites has produced adjacent alluvial deposits in ancient drainage systems. These are generally enriched in cassiterite. All rocks have been extensively lateritized during Tertiary peneplain formation; the laterite profile locally reaches depths in excess of 40 m below the original surface.
The C3 Pit contains the main lithium deposit. The lithium ore deposit occurs within a large (250 m wide) lithium enriched pegmatite. Spodumene in the Lithium ore zone can make up more than 50% of the rock with the remainder being largely quartz. Toward the northern end of C3 pit, a highly felspathic (K-feldspar) zone separates the high-grade lithium zone from the hanging wall amphibolite and the dolerite sill. Tantalum/tin and lithium ore body mineralization are conformable with the trends of the pegmatites both along strike and down dip.
Between C3 and C1 is the mining area referred to as C2. The pegmatite in this area dips approximately 40° west and has an intermediate composition with moderate lithium oxide Li2O values and moderate tantalum pentoxide (Ta2O5) values. This is in contrast to C1 and C3 which have large distinct zones of separate Li2O and Ta2O5 high grade.
At the southern end of the Central Lode pits is the C1 pit area. It contains the next largest concentration of high grade spodumene lithium mineralization after C3. The eastern footwall contact in the south of the C1 area dips 35 degrees west steepening toward the north and with depth. The internal grade domains in C1 parallel the eastern footwall contact. The immediate footwall is enriched in tantalum with typical accessory minerals tourmaline and apatite visible. Above are zones of lithium mineralization crosscut by deep weathering near surface altering the pegmatite to kaolin. Moving north the dip of the pegmatite shallows and the lithium domain at more than 1% Li2O is discontinuous.
Mineralogy
As stated above, internally the Greenbushes pegmatite displays up to five mineralogically defined zones; the Contact Zone, K-Feldspar (Potassium) Zone, Albite (Sodium) Zone, Mixed Zone and Spodumene (Lithium) Zone. Zones generally relate to multiple phases of intrusion and crystallization of the pegmatites.
The zones occur as a series of thick layers commonly with a lithium zone on the hanging wall or footwall, K-feldspar towards the hanging wall and a number of central albite zones. High-grade tantalum mineralization (more than 420 grams per tonne [g/t]) is generally confined to the Albite zone within the deposit. The Spodumene and K-Feldspar Zones typically have tantalum-tin grades of less than 100 ppm.
Major minerals are quartz, spodumene, albite and K-feldspar. Primary lithium minerals are spodumene, LiAlSi2O6 (~8% Li2O) and spodumene varieties kunzite and hiddenite. Minor lithium minerals include lepidolite (mica), amblygonite and lithiophilite (phosphates). Spodumene is hard (6.5-7) with an SG of 3.1-3.2. Highest concentrations (50%) of Spodumene occur in the C1 and C3 pits.
When spodumene is weathered and oxidized the lithium ions leach into the environment, the result is spodumene pegmatite weathered to clay. This is of little to no economic value to the current operation. Oxidation of the pegmatites has generally occurred in near-surface weathering or along selected structures internal to the pegmatites. Only the near-surface weathering is considered to materially affect the pegmatite from a process mineralogy standpoint.
Deposit Geometry
The Central Lode pegmatite deposit extends over a strike length of 3.5km north-south (local mine grid) and has previously been sub-divided for practical purposes into four sectors representing past and present open pit operations, known as (from north to south) the Cornwall (tantalum only), C3, C2 and C1 pits.
However, recent drilling has demonstrated broad continuity at depth over the entire strike length and it can be considered as a single entity for resource and reserve modeling purposes. The overall pegmatite zone dips grid west at approximately 40º, has an overall thickness approaching 300m, and has been interpreted to a depth of over 600m below surface.
The Kapanga deposit lies some 300m east of Central Lode. It has been interpreted over 1.8km of strike and trends sub-parallel to Central Lode, dipping typically at 40-50º to the west-southwest, but steepening to 60º in the south. It comprises several stacked sub-parallel lodes of varying thicknesses, as well as smaller pods, with an overall thickness of approximately 150m, including intervening wallrocks. Mineralised pegmatite has been intersected by drilling to a depth of 450m below surface.
